This invention generally relates to the molding of elastomeric materials and in particular to methods and compositions for reducing fouling of a mold caused by vulcanization of rubber compounds in the mold.
A rubber compound is often vulcanized in a mold to form a product, such as a tire. During vulcanization, materials, such as sulfur, zinc salts, oils, and waxes tend to migrate from the rubber compound and deposit on the mold. Repeated vulcanization of rubber compounds in the mold causes the material to build up on the mold. This buildup of material is commonly referred to as "mold fouling".
Mold fouling reduces the aesthetic value of a molded product, and can degrade the physical properties of the molded product.
In order to prevent the foregoing adverse effects of mold fouling, a mold has to be periodically cleaned. In the tire industry, an automobile tire mold is typically cleaned every thirty (30) days. Since an automobile tire mold is typically used to vulcanize about 4 tires per hour, 24 hours per day, which is about 100 tires per day, an automobile tire mold is typically cleaned after 3000 tires have been vulcanized in the tire mold. For other rubber products, the number of products that can be vulcanized in a mold before the mold is cleaned may be less.
A mold is typically cleaned by either particle blasting or immersion in a cleaning liquid. Particles commonly used in the blasting method include sand, carbon dioxide pellets, walnut shells, and beads composed of plastic, glass, ceramic, or metal. Cleaning liquids commonly used in the immersion method include strong acids, such as hydrochloric acid, sulfuric acid, and nitric acid, or strong alkalis, such as caustic soda, or solvents.
The foregoing cleaning methods are rather costly and tend to damage and/or contaminate molds. Moreover, production is lost when molds are being cleaned. Accordingly, attempts have been made to reduce the frequency with which molds must be cleaned.
Release agents have been developed that are comprised of waxes, fatty acids, polytetrafluoroethylene (PTFE), and metallic soaps. These release agents migrate to the surface of the rubber composition during vulcanization to form a lubricating film between the rubber composition and the mold. The release agents prevent the rubber composition from sticking to the mold, but do not abate mold fouling.
Molds have also been plated with chrome and coated with polytetrafluoroethylene (PTFE) and other materials to reduce mold fouling. These techniques, however, have had only limited success in reducing mold fouling.
Based on the foregoing, there is a need in the art for a method for reducing mold fouling. The present invention is directed to such a method.